Transmission channels in a cellular telephone system are typically selected using a sequential channel selection process. During operation, channels may be allocated by first creating a list of potential channels, then sequentially selecting channels from the created list. The list is usually arranged as an ordered list, for example, a priority que arrangement or a table. The channel selection protocol usually starts by selecting the first channel and determining if the selected channel is available to be assigned for transmission. If the first channel is available, it is selected for transmission. If the first channel is not available, the second channel's availability is checked. This process is repeated until an available channel is found, or until all of the channels in the list have been checked.
The above-described allocation method does not take the signal's quality or historical failure rates into account when selecting a channel. In general, channels having a higher signal quality level transmit voice and other information with better clarity and overall performance than channels having a lower signal quality level. Also, channels having high historical failure rates have a greater probability of failure. Further, when using a sequential channel selection protocol, channels at the head of the que tend to be selected much more often than channels near the end of the que. The result is that transmission circuits associated with the first few channels wear out from overuse more quickly than other transmission circuits.
A method and apparatus for allocating transmission channels in a cellular system based on channel quality but not historical failure rates is disclosed in a related application entitled Method and Apparatus for Allocating Channels in a Cellular System, Ser. No. 08/373,927 (Inventor: Bhanu Durvasula). That invention provides a method and apparatus for allocating channels in a cellular system by selecting transmission channels based only on a measured signal quality level of each channel. The method for allocating channels includes the steps of providing transmission channels suitable for transmission over a cell of the cellular system, defining a quality channel pool having a range of acceptable signal quality levels, measuring the signal quality level for each channel, and placing into the quality channel pool each channel having a signal quality level within the acceptable range. Channels are preferably selected from a round robin selection technique.
Although the above-described invention creates a hierarchy of preferred channels based on calculated signal qualities and increases the useful life of channel transmission circuitry, there is still need for even further improvement. For example, there is a need for a method that takes into consideration during channel selection the historical failure rates of a high quality channel. This would avoid the problem of calls being assigned to high quality but historically problematic channels only to have the calls disconnected, or connected with a poor quality of service. There is a further need for a channel allocation scheme that allows the operator to monitor the most problematic channels after each channel assignment. There is an even further need for a method whereby channels possessing the highest quality signals do not remain in their respective quality subpools even if they possess a history of pre-assignment and/or post-assignment failures. There is still further need of a mechanism to assign problematic channels a lower allocation priority even if their signal quality is high, whereby an operator could eliminate highly problematic channels from subsequent channel assignments, rectify the problematic area, then place the channel back into a quality subpool for future assignment.